1. Field of the Invention
The present invention relates to a fuel injection device for injecting fuel into an intake passage of an engine, and specifically relates to a fuel injection device in which a solenoid valve opens with a predetermined timing based on an injection command signal that has been output in accordance with the crank angle position of the engine. The device is suitable for use in, for example, all-purpose engines used as sources of power in equipment such as agricultural, construction, and hand-held operating equipment, as well as small vehicles.
2. Description of the Related Art
Fuel injection devices that inject/supply pressurized fuel to the intake passage of an engine are well known, as described, for example, in JP (Kokai) 2001-193610. With this type of fuel injection device, a drive current computed by an electronic controller is applied as a fuel injection signal to the coil of a solenoid valve (injector) for each engine rotation, and a needle valve is raised while the fixed iron core is magnetized to open the valve and inject the fuel for a predetermined period of time.
FIG. 3 is a graph showing the relationship between engine speed and fuel flow rate in this type of fuel injection device. The graph shows that sufficient fuel can be supplied to achieve the required flow rate in an engine under conditions in which the engine speed is not high. It can also be seen, however, that a time lag between the start of the current flow and the reaching of the valve-opening voltage is created by the slow rise of the drive current because of the inductance in the coil of the solenoid valve.
This creates a problem in that the per-cycle effective injection time during which the solenoid valve opens and fuel is sprayed decreases with increased engine speed, that the injection period, even when set earlier, still does not match the supply of fuel, and the feed rate is insufficient relative to the flow rate required by the engine during high-speed operation, and that engine operation tends to be unbalanced.
In response to this problem, there is proposed in JP (Kokai) 11-107836 a fuel injection device provided with an alternating-current generator driven by engine rotation, and a rectifier for rectifying the motor output. These elements constitute a drive power-source circuit for increasing the magnitude of the drive voltage applied to the solenoid valve as the engine speed increases from start-up to high-speed operation. The device is further provided with an injector trigger circuit for allowing a drive current to flow from the drive power-source circuit to the solenoid valve.
Providing an alternating-current generator to the drive power-source circuit allows the drive voltage to be increased in accordance with increased engine speed, the opening of the solenoid valve to be accelerated, and a long effective injection time to be maintained. Consequently, it is possible to increase the fuel injection rate in accordance with increased engine speed and to more easily provide the flow rate required by the engine even during high-speed operation.
However, this fuel injection device requires that a rectifier, an injection trigger circuit, and an electronic controller as injection command generating means be placed between the alternating-current generator and the solenoid valve, and the final drive current is also output as a direct current. Consequently, using a higher voltage for the drive current fails to achieve a substantially rapidly rising voltage, and the effect of reducing the injection delay time is insufficient. In addition, increasing the number of parts that constitute the fuel injection device makes the device configuration more complicated and tends to increase costs.
The present invention is aimed at resolving the aforementioned problems and providing a fuel injection device for injecting/supplying fuel to an engine inexpensively and at the fuel flow rate required for high-speed operation.